LIBRARY 

OF  THE 

UNIVERSITY  OF  CALIFORNIA. 

RECEIVED    BY   EXCHANGE 

Class 


J 


HENRY  DRAPKR  MEMORIAL. 


FIRST    ANNUAL    REPORT 


OF     THE 


PHOTOGRAPHIC  STUDY  OF  STELLAR  SPECTRA 


<  OMH'CTED    AT    THE 


HARVARD  COLLEGE  OBSERVATORY, 


EDWARD    0.    PICKERING,   DIKECTOK. 


WITH    PLATE. 


\.MBRIDGK: 
JOHN     WILSON     AND     SOX, 


WmDttstto  }Jrcs8, 

1887. 


HENRY  DRAPER  MEMORIAL. 


FIRST    ANNUAL   REPORT 


OF    THE 


PHOTOGRAPHIC  STUDY  OF  STELLAR  SPECTRA 


CONDUCTED    AT    THE 


HARVARD  COLLEGE  OBSERVATORY, 


EDWARD    C.    PICKERING,  DIHECTOR. 


WITH    PLATE. 


THR 

rnriv       TY 


CAMBRIDGE: 
JOHN    WILSON    AND    SON, 


1887. 


Yt 

v.) 


*  *       *        »    i  *  *     t      t  ,      .       . 

•.:,.:•• 


HENRY    DRAPER    MEMORIAL. 


FIRST  ANNUAL  REPORT. 

DR.  HENRY  DRAPER,  in  1872,  was  the  first  to  photograph  the  lines  of  a  stellar 
spectrum.  His  investigation,  pursued  for  many  years  with  great  skill  and  ingenuity, 
was  most  unfortunately  interrupted  in  1882  by  his  death.  The  recent  advances  in 
dry-plate  photography  have  vastly  increased  our  powers  of  dealing  with  this  subject. 
Early  in  1886,  accordingly,  Mrs.  Draper  made  a  liberal  provision  for  carrying  on  this 
investigation  at  the  Harvard  College  Observatory,  as  a  memorial  to  her  husband. 
The  results  attained  are  described  below,  and  show  that  an  opportunity  is  open  for 
a  very  important  and  extensive  investigation  in  this  branch  of  astronomical  physics. 
Mrs.  Draper  has  accordingly  decided  greatly  to  extend  the  original  plan  of  work, 
and  to  have  it  conducted  on  a  scale  suited  to  its  importance.  The  attempt  will  be 
made  to  include  all  portions  of  the  subject,  so  that  the  final  results  shall  form  a  com- 
plete discussion  of  the  constitution  and  conditions  of  the  stars,  as  revealed  by  their 
spectra,  so  far  as  present  scientific  methods  permit.  It  is  hoped  that  a  greater 
advance  will  thus  be  made  than  if  the  subject  was  divided  among  several  institu- 
tions, or  than  if  a  broader  range  of  astronomical  study  was  attempted.  It  is 
expected  that  a  station  to  be  established  in  the  southern  hemisphere  will  permit  the 
work  to  be  extended  so  that  a  similar  method  of  study  may  be  applied  to  stars  in 
all  parts  of  the  sky.  The  investigations  already  undertaken,  and  described  below 
more  in  detail,  include  a  catalogue  of  the  spectra  of  all  stars  north  of — 24°  of  the 
sixth  magnitude  and  brighter,  a  more  extensive  catalogue  of  spectra  of  stars  brighter 
than  the  eighth  magnitude,  and  a  detailed  study  of  the  spectra  of  the  bright  stars. 
This  last  will  include  a  classification  of  the  spectra,  a  determination  of  the  wave 
lengths  of  the  lines,  a  comparison  with  terrestrial  spectra,  and  an  application  of 
the  results  to  the  measurement  of  the  approach  and  recession  of  the  stars.  A  spe- 
cial photographic  investigation  will  also  be  undertaken  of  the  spectra  of  the  banded 
stars,  and  of  the  ends  of  the  spectra  of  the  bright  stars.  The  instruments  employed 


4  HENRY   DRAPER   MEMORIAL. 

are  an  8-inch  Voigtlander  photographic  lens  reground  by  Alvan  Clark  and  Sons,  and 
Dr.  Draper's  11-inch  photographic  lens,  for  which  Mrs.  Draper  has  provided  a  new 
mounting  and  observatory.  The  15-inch  refractor  belonging  to  the  Harvard  Col- 
lege Observatory  has  also  been  employed  in  various  experiments  with  a  slit  spectro- 
scope, and  is  again  being  used  as  described  below.  Mrs.  Draper  has  decided  to  send 
to  Cambridge  a  28-inch  reflector  and  its  mounting,  and  a  15-inch  mirror,  which  is 
one  of  the  most  perfect  reflectors  constructed  by  Dr.  Draper,  and  with  which  his 
photograph  of  the  Moon  was  taken.  The  first  two  instruments  mentioned  above 
have  been  kept  at  work  during  the  first  part  of  every  clear  night  for  several 
months.  It  is  now  intended  that  at  least  three  telescopes  shall  be  used  during  the 
whole  night,  until  the  work  is  interrupted  by  daylight. 

The  spectra  have  been  produced  by  placing  in  front  of  the  telescope  a  large  prism, 
thus  returning  to  the  method  originally  employed  by  Fraunhofer  in  the  first  study 
of  stellar  spectra.  Four  15°  prisms  have  been  constructed,  the  three  largest  having 
clear  apertures  of  nearly  eleven  inches,  and  the  fourth  being  somewhat  smaller.  The 
entire  weight  of  these  prisms  exceeds  a  hundred  pounds,  and  they  fill  a  brass  cubical 
box  a  foot  on  each  side.  The  spectrum  of  a  star  formed  by  this  apparatus  is  ex- 
tremely narrow  when  the  telescope  is  driven  by  clockwork  in  the  usual  way.  A 
motion  is  accordingly  given  to  the  telescope  slightly  differing  from  that  of  the  earth 
by  means  of  a  secondary  clock  controlling  it  electrically.  The  spectrum  is  thus 
spread  into  a  band,  having  a  width  proportional  to  the  time  of  exposure  and  to  the 
rate  of  the  controlling  clock. 

This  band  is  generally  not  uniformly  dense.  It  exhibits  lines  perpendicular  to 
the  refracting  edge  of  the  prism,  such  as  are  produced  in  the  field  of  an  ordinary 
spectroscope  by  particles  of  dust  upon  the  slit.  In  the  present  case,  these  lines 
may  be  due  to  variations  in  the  transparency  of  the  air  during  the  time  of  ex- 
posure, or  to  instrumental  causes,  such  as  irregular  running  of  the  driving  clock, 
or  slight  changes  in  the  motion  of  the  telescope,  resulting  from  the  manner  in 
which  its  polar  axis  is  supported.  These  instrumental  defects  may  be  too  small  to 
be  detected  in  ordinary  micrometric  or  photographic  observations,  and  still  suffi- 
cient to  affect  the  photographs  just  described. 

A  method  of  enlargement  has  been  tried  which  gives  very  satisfactory  results, 
and  removes  the  lines  above  mentioned  as  defects  in  the  negatives.  A  cylindrical 
lens  is  placed  close  to  the  enlarging  lens,  with  its  axis  parallel  to  the  length  of  the 
spectrum.  In  the  apparatus  actually  employed,  the  length  of  the  spectrum,  and 
with  it  the  dispersion,  is  increased  five  times,  while  the  breadth  is  made  in  all 
cases  about  four  inches.  The  advantage  of  this  arrangement  is,  that  it  greatlv 


FIRST   ANNUAL    REPORT.  5 

reduces  the  difficulty  arising  from  the  feeble  light  of  the  star.  Until  very  lately, 
the  spectra  in  the  original  negatives  were  made  very  narrow,  since  otherwise  the 
intensity  of  the  starlight  would  have  been  insufficient  to  produce  the  proper  decom- 
position of  the  silver  particles.  The  enlargement  being  made  by  daylight,  the  vast 
amount  of  energy  then  available  is  controlled  by  the  original  negative,  the  action  of 
which  may  be  compared  to  that  of  a  telegraphic  relay.  The  copies  therefore  repre- 
sent many  hundred  times  the  original  energy  received  from  the  stars.  If  care  is  not 
taken,  the  dust  and  irregularities  of  the  film  will  give  trouble,  each  foreign  particle 
appearing  as  a  fine  spectral  line. 

Other  methods  of  enlargement  have  been  considered,  and  some  of  them  tried, 
with  the  object  of  removing  the  irregularities  of  the  original  spectra  without  intro- 
ducing new  defects.  For  instance,  the  sensitive  plate  may  be  moved  during  the 
enlargement  in  the  direction  of  the  spectral  lines ;  a  slit  parallel  to  the  lines  may 
be  used  as  the  source  of  light,  and  the  original  negative  separated  by  a  small  inter- 
val from  the  plate  used  for  the  copy ;  or  two  cylindrical  lenses  may  be  used,  with 
their  axes  perpendicular  to  each  other.  In  some  of  these  ways  the  lines  due  to 
dust  might  either  be  avoided  or  so  much  reduced  in  length  as  not  to  resemble  the 
true  lines  of  the  spectrum. 

The  15-inch  refractor  is  now  being  used  with  a  modification  of  the  apparatus 
employed  by  Dr.  Draper  in  his  first  experiments,  —  a  slit  spectroscope  from  which 
the  slit  has  been  removed.  A  concave  lens  has  been  substituted  for  the  collimator 
and  slit,  and,  besides  other  advantages,  a  great  saving  in  length  is  secured  by  this 
change.  It  is  proposed  to  apply  this  method  to  the  28-inch  reflector,  thus  utilizing 
its  great  power  of  gathering  light. 

The  progress  attained  is  best  shown  by  the  accompanying1  Plate.  Figure  1 
is  a  direct  copy  of  the  spectra  obtained  in  1882.  They  were  made  by  placing  a 
30"  prism  in  front  of  a  Voigtlander  lens  having  an  aperture  of  two  inches  and  a  focal 
length  of  seven  inches.  The  exposures  lasted  for  about  five  minutes,  and  no  clock- 
work was  used.  The  instrument  was  directed  successively  to  a  Lyrce,  a  Aquilce, 
a  Boolis,  and  yS  and  y  Ursa;  Minoris,  all  of  whose  spectra  appear  in  the  figure. 
Figure  2  represents  the  spectra  of  £  Ursce  Majoris,  and  of  the  adjacent  fifth- 
magnitude  star.  It  was  taken  with  the  8-inch  Voigtlander  lens,  with  an  exposure 
of  five  minutes,  and  illustrates  the  size  of  spectra  used  in  preparing  the  catalogue 
of  spectra  of  the  brighter  stars.  One  or  two  hundred  of  these  spectra  are  some- 
times photographed  upon  a  single  plate.  Figure  3  was  obtained  with  the  Draper 
11-inch  telescope,  with  two  prisms.  It  represents  the  spectrum  of  a  Lyra;,  and  was 
taken  on  November  5,  188G,  with  an  exposure  of  59  minutes.  Figure  4  was  taken 


6  HENRY   DRAPER   MEMORIAL. 

on  January  21,  1887,  in  50  minutes,  with  four  prisms  attached  to  the  same  instru- 
ment. It  represents  the  spectrum  of  ft  Geminontm.  All  of  these  figures  are  made 
by  direct  printing  from  the  original  negatives.  A  positive  was  first  made  by  laying 
the  original  plates  in  turn  on  a  sensitive  plate  and  exposing  it  to  the  light.  A  nega- 
tive was  formed  from  this  in  the  same  manner,  which  by  the  Albertype  process  was 
reproduced  on  paper.  Figure  5  illustrates  the  method  of  enlargement  described 
above.  It  represents  the  portions  of  Figure  4  contained  between  the  points  marked 
m  and  n.  The  entire  spectrum  would  therefore  have  a  length  nearly  double  that 
here  represented.  A  less  perfect  spectrum  of  the  same  star  was  obtained  on  Jan- 
uary 12,  1887.  A  portion  of  its  enlargement  is  given  in  Figure  6,  adjacent  to 
Figure  5.  Nearly  all  the  lines  of  Figure  5  are  shown  less  clearly  in  Figure  6. 
Some  of  the  remainder  are  due.  to  the  irregularities  of  the  film  described  above, 
but  they  are  probably  too  minute  to  be  visible  in  the  paper  prints. 

The  results  to  be  derived  from  the  large  number  of  photographs  already  obtained 
can  only  be  stated  after  a  long  series  of  measurements  and  a  careful  reduction  and 
discussion  of  them.  An  inspection  of  the  plates,  however,  shows  some  points  of  inter- 
est. A  photograph  of  a  Ct/yni,  taken  November  26,  1886,  shows  that  the  H  line  is 
double,  its  two  components  having  a  difference  in  wave  length  of  about  one  ten- 
millionth  of  a  millimeter.  A  photograph  of  o  Ceti  shows  that  the  lines  G  and  h  are 
bright,  as  are  also  four  of  the  ultra  violet  lines  characteristic  of  spectra  of  the  first 
type.  The  II  and  K  lines  in  this  spectrum  are  dark,  showing  that  they  probably  do 
not  belong  to  that  series  of  lines.  The  star  near  •£  Orionis,  discovered  by  Gore  in 
December,  1885,  gives  a  similar  spectrum,  which  affords  additional  evidence  that 
it  is  a  variable  of  the  same  class  as  o  Ceti.  Spectra  of  Sirius  show  a  large  number 
of  faint  lines  besides  the  well-known  broad  lines. 

The  dispersion  employed  in  any  normal  map  of  the  spectrum  may  be  expressed 
by  its  scale,  that  is,  by  the  ratio  of  the  wave  length  as  represented  to  the  actual 
wave  length.  It  will  be  more  convenient  to  divide  these  ratios  by  one  million,  to 
avoid  the  large  numbers  otherwise  involved.  If  one  millionth  of  a  millimeter  is 
taken  as  the  unit  of  wave  length,  the  length  of  this  unit  on  the  rnnp  in  millimeters 
will  give  the  same  measure  of  the  dispersion  as  that  just  described.  When  the  map 
is  not  normal,  the  dispersion  of  course  varies  in  different  parts.  It  increases  rapidly 
towards  the  violet  end  when  the  spectrum  is  formed  by  a  prism.  Accordingly,  in 
this  case  the  dispersion  given  will  be  that  of  the  point  whose  wave  length  is  400. 
This  point  lies  near  the  middle  of  the  photographic  spectrum  when  a  prism  is  used, 
and  is  not  far  from  the  H  line.  The  dispersion  may  accordingly  be  found  with 
sufficient  accuracy  by  measuring  the  interval  between  the  H  and  K  lines,  and 


FIKST   ANNUAL   REPORT.  ( 

dividing  the  result  in  millimeters  by  3.4,  since  the  difference  in  their  wave  lengths 
equals  this  quantity.  The  following  examples  serve  to  illustrate  the  dispersion 
expressed  in  this  way  :  Angstrom,  Cornu,  10 ;  Draper,  photograph  of  normal  solar 
spectrum,  3.1  and  5.2;  Rowland,  23,  33,  and  46;  Draper,  stellar  spectra,  0.16; 
Huggins,  0.1.  Fig.  1,  O.OG ;  Fig.  2,  0.10;  Fig.  3,  0.63;  Fig.  4,  1.3;  Figs.  5 
and  6,  0.5. 

The  most  rapid  plates  are  needed  in  this  work,  other  considerations  being  gen- 
erally of  less  importance.  Accordingly  the  Allen  and  Rowell  Extra  Quick  plates 
have  been  used  until  recently.  It  was  found,  however,  that  they  were  surpassed  by 
the  Seed  Plates  No.  21,  which  were  accordingly  substituted  for  them  early  in 
December.  Recognizing  the  importance  of  supplying  this  demand  for  the  most  sen- 
sitive plates  possible,  the  Seed  Company  have  recently  succeeded  in  making  still  more 
sensitive  plates,  which  we  are  now  using.  The  limit  does  not  seem  to  be  reached 
even  yet.  Plates  could  easily  be  handled  if  the  sensitiveness  were  increased  tenfold. 
A  vast  increase  in  the  results  may  be  anticipated  with  each  improvement  of  the 
plates  in  this  respect.  Apparatus  for  testing  plates,  which  is  believed  to  be  much 
more  accurate  than  that  ordinarily  employed,  is  in  course  of  preparation.  It  is  ex- 
pected that  a  very  precise  determination  will  be  made  of  the  rapidity  of  the  plates 
employed.  Makers  of  very  rapid  plates  are  invited  to  send  specimens  for  trial. 

The  photographic  work  has  been  done  by  Mr.  W.  P.  Gerrish,  who  has  also  ren- 
dered important  assistance  in  other  parts  of  the  investigation.  He  has  shown  great 
skill  in  various  experiments  which  have  been  tried,  and  in  the  use  of  various  novel 
and  delicate  instruments.  Many  of  the  experimental  difficulties  could  not  have  been 
overcome  but  for  the  untiring  skill  and  perseverance  of  Mr.  George  B.  Clark,  of 
the  firm  of  Alvan  Clark  and  Sons,  by  whom  all  the  large  instruments  have  been 
constructed. 

The  progress  of  the  various  investigations  which  are  to  form  a  part  of  this  work  is 
given  below  :  — 

1.  Catalogue  of  Spectra  of  Bright  Stars.  —  This  is  a  continuation  of  the  work 
undertaken  with  the  aid  of  an  appropriation  from  the  Bache  Fund,  and  described 
in  the  Memoirs  of  the  American  Academy,  Vol.  XI.  p.  210.  The  8-inch  telescope 
is  used,  each  photograph  covering  a  region  "10°  square.  The  exposures  for  equatorial 
stars  last  for  five  minutes,  and  the  rate  of  the  clock  is  such  that  the  spectra  have  a 
width  of  about  0.1  cm.  The  length  of  the  spectra  is  about  1.2  cm.  for  the  brighter, 
and  0.6  cm.  for  the  fainter  stars.  The  dispersion  on  the  scale  proposed  above  is  0.1. 
The  spectra  of  all  stars  of  the  sixth  magnitude  and  brighter  will  generally  be  found 


8  HENRY   DRAPER    MEMORIAL. 

upon  these  plates,  except  in  the  case  of  red  stars.  Many  fainter  blue  stars  also 
appear.  Three  or  four  exposures  are  made  upon  a  single  plate.  The  entire  sky 
north  of  — 24°  would  be  covered  twice,  according  to  this  plan,  with  180  plates  and 
690  exposures.  It  is  found  preferable  in  some  cases  to  make  only  two  exposures  ; 
and  when  the  plate  appears  to  be  a  poor  one,  the  work  is  repeated.  The  number 
of  plates  is  therefore  increased.  Last  summer  the  plates  appeared  to  be  giving 
poor  results.  Dust  on  the  prisms  seemed  to  be  the  explanation  of  this  difficulty. 
Many  regions  were  reobserved  on  this  account.  The  first  cycle,  covering  the  entire 
sky  from  zero  to  twenty-four  hours  of  right  ascension,  has  been  completed.  The 
work  will  be  finished  during  the  coming  year  by  a  second  cycle  of  observations, 
which  has  already  been  begun.  The  first  cycle  contains  257  plates,  all  of  which 
have  been  measured,  and  a  large  part  of  the  reduction  completed.  8,313  spectra 
have  been  measured  on  them,  nearly  all  of  which  have  been  identified,  and  the 
places  of  a  greater  portion  of  the  stars  brought  forward  to  the  3-ear  1900,  and 
entered  in  catalogue  form.  In  the  second  cycle,  64  plates  have  been  taken,  and 
about  as  many  more  will  be  required.  51  plates  have  been  measured  and  identified, 
including  2,974  spectra.  A  study  of  the  photographic  brightness  and  distribution 
of  the  light  in  the  spectra  will  also  be  made. 

The  results  will  be  published  in  the  form  of  a  catalogue  resembling  the  Photo- 
metric Catalogue  given  in  Volume  XIV.  of  the  Annals  of  Harvard  College  Observa- 
tory. It  will  contain  the  approximate  place  of  each  star  for  1900,  its  designation, 
the  character  of  the  spectrum  as  derived  from  each  of  the  plates  iu  which  it 
was  photographed,  the  references  to  these  plates,  and  the  photographic  brightness  of 
the  star. 

2.  Catalogue  of  Spectra  of  Faint  Stars.  —  This  work  resembles  the  preceding,  but 
is  much  more  extensive.  The  same  instrument  is  used,  but  each  region  has  an 
exposure  of  an  hour,  the  rate  of  the  clock  being  such  that  the  width  of  the  spectrum 
will  be  as  before  0.1  cm.  Many  stars  of  the  ninth  magnitude  will  thus  be  included, 
and  nearly  all  brighter  than  the  eighth.  In  one  case,  over  three  hundred  spectra  are 
shown  on  a  single  plate.  This  work  has  been  carried  on  only  in  the  intervals  when 
the  telescope  was  not  needed  for  other  purposes.  99  plates  have  however  been 
obtained,  and  on  these  4,442  spectra  have  been  measured.  It  is  proposed  to  com- 
plete the  equatorial  zones  first,  gradually  extending  the  work  northward.  In  all, 
15,729  spectra  of  bright  and  faint  stars  have  been  measured. 

8.    Detailed  Study  of  the  Spectra  of  the  Brighter  Stars.  —  This  work  has  been  carried 

.on  with  the  11-inch  photographic  telescope  used  by  Dr.  Draper  in  his  later  researches. 

A  wooden  observatory  was  constructed  about  20  feet  square.     This  was  surmounted 


FIRST   ANNUAL    REPORT.  9 

by  a  dome  having  a  clear  diameter  of  18  feet  on  the  inside.  The  dome  had  a  wooden 
frame,  sheathed  and  covered  with  canvas.  It  rested  on  eight  cast-iron  wheels,  and 
was  easily  moved  by  hand,  the  power  being  directly  applied.  Work  was  begun 
upon  it  in  June,  and  the  first  observations  were  made  with  the  telescope  in  October. 
Two  prisms  were  formed  by  splitting  a  thick  plate  of  glass  diagonally.  These  gave 
such  good  results  that  two  others  were  made  in  the  same  way,  and  the  entire  battery 
of  four  prisms  is  ordinarily  used.  The  safety  and  convenience  of  handling  the  prisms 
is  Tjreatly  increased  by  placing  them  in  square  brass  boxes,  each  of  which  slides  into 
place  like  a  drawer.  Any  combination  of  the  prisms  may  thus  be  employed.  As  is 
usual  in  such  an  investigation,  a  great  variety  of  difficulties  have  been  encountered, 
and  the  most  important  of  them  have  now  been  overcome. 

4.  Faint  Stellar  Spectra.  —  The  28-inch  reflector  will  be  used  for  the  study  of  the 
spectra  of  the  faint  stars,  and  also  for  the  fainter  portions  near  the  ends  of  the 
spectra  of  the  brighter  stars.     The  form  of  spectroscope  mentioned  above,  in  which 
the  collirnator  and  slit  are  replaced  by  a  concave  lens,  will  be  tried.     The  objects 
to  be  examined  are,  first,  the  stars  known  to  be  variable,  with  the  expectation  that 
some  evidence  may  be  afforded  of  the  cause  of  the  variation.     The  stars  whose 
spectrum  is  known  to  be  banded,  to  contain  bright  lines,  or  to  be  peculiar  in  other 
respects,  will  also  be  examined  systematically.     Experiments  will  also  be  tried  with 
orthochromatic  plates  and  the  use  of  a  colored  absorbing  medium,  in  ordef  to  photo- 
graph the  red  portions  of  the  spectra  of  the  bright  stars.     Quartz  will  also  be  tried 
to  extend  the  images  towards  the  ultra  violet. 

5.  Absorption  Spectra.  —  The  ordinary  form  of   comparison    spectrum   cannot   be 
employed    on    account  of  the  absence   of  a  slit.     The    most  promising    method   of 
determining  the  wave  lengths  of  the  stellar  spectra  is  to  interpose  some  absorbent 
medium.     Experiments  are  in  progress  with  hyponitric  fumes  and  other  substances. 
A  tank  containing  one  of  these  materials  is  interposed,  and  the  spectra  photographed 
through  it.     The  stellar  spectra  will  then  be  traversed  by  lines  resulting  from  the 
absorption   of  the  media  thus   interposed,  and,  after  their  wave  lengths   are  once 
determined,  they  serve  as  a  precise  standard   to  which   the   stellar  lines  may  be 
referred.     The  absorption  lines  of  the    terrestrial  atmosphere  would   form  the   best 
standard  for  this  purpose  if  those  which  are  sufficiently  fine  can  be  photographed. 

C.  Ware  Lengths.  —  The  determination  of  the  wave  lengths  of  the  lines  in  the 
stellar  spectra  will  form  an  important  part  of  the  work  which  has  not  yet  been 
begun.  The  approximate  wave  lengths  can  readily  be  found  from  a  comparison 
with  the  solar  spectrum,  a  sufficient  number  of  solar  lines  being  present  in  most 
stellar  spectra.  As  a  difference  of  one  ten-millionth  of  a  millimeter  in  wave  length 


10  HENRY    DEAFER   MEMORIAL. 

exceeds  half  a  millimeter  in  Figures  5  and  6  of  the  accompanying  Plate,  the  readings 
may  be  made  with  considerable  accuracy  by  a  simple  inspection.  For  greater 
precision  special  precautions  are  necessary  on  account  of  the  deviation  caused  by 
the  approach  and  recession  of  the  stars.  The  deviation  found  by  Dr.  Huggins  in  the 
case  of  Sinus  would  correspond  to  a  change  in  the  position  of  the  lines  of  Figures 
5  and  6  of  about  half  a  millimeter.  If,  then,  satisfactory  results  are  obtained  in 
the  preceding  investigation,  the  motion  of  the  stars  can  probably  be  determined 
with  a  high  degree  of  precision.  The  identification  of  the  lines  with  those  of 
terrestrial  substances  will  of  course  form  a  part  of  the  work,  but  the  details  will  be 
considered  subsequently. 

From  the  above  statement  it  will  be  seen  that  photographic  apparatus  has  been 
furnished  on  a  scale  unequalled  elsewhere.  But  what  is  more  important,  Mrs.  Draper 
has  not  only  provided  the  means  for  keeping  these  instruments  actively  employed, 
several  of  them  during  the  whole  of  every  clear  night,  but  also  of  reducing  the 
results  by  a  considerable  force  of  computers,  and  of  publishing  them  in  a  suitable 
form.  A  field  of  work  of  great  extent  and  promise  is  open,  and  there  seems  to  be 
an  opportunity  to  erect  to  the  name  of  Dr.  Henry  Draper  a  memorial  such  as 
heretofore  no  astronomer  has  received.  One  cannot  but  hope  that  such  an  example 
may  be  imitated  in  other  departments  of  astronomy,  and  that  hereafter  other  names 
may  be  commemorated,  not  by  a  needless  duplication  of  unsupported  observatories, 
but  by  the  more  lasting  monuments  of  useful  work  accomplished. 

EDWARD   C.  PICKERING, 

Director  of  Harvard  College  Observatory. 
CAMBRIDGE,  MASS.,  U.  S.  A., 
March  1, 18$7. 


HENRY  DRAPKH   MKMOIMAI, 


SKCONI)    ANN!  AI,    UKl'OKT 


01       T1IK 


PUOTOGIUPIIIC  STUDY  OF  STELLAR  SPECTRA 


!•;:>   AT  TIIK 


IIAUVAUD  C()LLE(ii:   OBSERVATORY, 


EDWARD    C.    PK'KKIMX*;,    DIRECTOR. 


WITH    TWO    PLATES. 


CAM  HIM  DGE: 
JOHN    WILSON     ANIi    SON, 

WmDtrsitu  IJrrss. 

L888. 


HENRY  DRAPER  MEMORIAL. 


SECOND    ANNUAL   REPORT 


OF    THE 


PHOTOGRAPHIC  STUDY  OF  STELLAR  SPECTRA 


CONDUCTED    AT   THE 


HARVARD  COLLEGE  OBSERVATORY, 


EDWARD    C.   PICKERING,  DIRECTOR. 


WITH    TWO    PLATES. 


CAMBRIDGE: 

JOHN    WILSON    AND    SON, 

Slmbtrsito 

1888. 


HENRY    DRAPER    MEMORIAL. 


SECOND   ANNUAL   REPORT. 

THE  additional  facilities  provided  by  Mrs.  Draper  have  permitted  a  considerable 
extension  of  this  research  during  the  past  year.  The  11-inch  refractor  belonging 
to  Dr.  Draper,  and  the  8-inch  photographic  telescope  provided  by  the  Bache  Fund, 
have  been  kept  at  work  throughout  every  clear  night  The  28-inch  and  15-inch 
reflectors  constructed  by  Dr.  Draper  have  been  moved  to  Cambridge,  and  the  first 
of  these  instruments  is  placed  in  a  building  surmounted  by  a  dome  constructed 
for  the  purpose.  Experiments  are  now  in  progress  with  it,  and  it  will  probably 
soon  be  employed  regularly.  Four  assistants  take  part  in  making  the  photographs, 
one  of  whom  comes  to  the  Observatory  every  clear  night  about  midnight,  and  keeps 
the  8-inch  and  11-inch  telescopes  in  use  until  interrupted  by  the  morning  twilight 
Five  ladies  have  been  employed  in  the  measurements  and  reductions. 

The  various  investigations  now  in  progress  are  described  in  detail  below.  The 
first  three  of  these,  including  the  photographic  work  of  the  8-inch  and  11-inch  tele- 
scopes, will  be  finished  in  about  a  year.  It  is  accordingly  proposed  in  the  autumn 
of  1889  to  send  an  expedition  to  the  southern  hemisphere,  probably  to  Peru,  and 
there  complete  the  work  to  the  south  pole.  As  only  about  one  quarter  part  of  the 
sky  is  too  far  south  to  be  conveniently  observed  at  Cambridge,  it  is  expected  that 
the  photographs  needed  to  cover  this  portion  of  the  sky  could  be  obtained  in  two 
years.  Each  investigation  could  thus  be  extended  to  all  parts  of  the  sky  upon 
the  same  system. 

The  present  location  of  the  various  instruments  is  shown  in  the  Frontispiece 
to  this  Report.  It  represents  the  rear  of  the  Harvard  College  Observatory,  and 
is  taken  from  the  top  of  a  cottage  to  the  northwest  of  the  main  building.  Of 
the  three  domes  in  the  foreground  the  right-hand  one  contains  the  11-inch  pho- 
tographic telescope,  and  the  central  one  the  28-inch  reflector.  The  15-inch  re- 
flector has  been  attached  to  the  tube  standing  between  the  two  domes.  The 


4  HENRY   DRAPER   MEMORIAL. 

8-inch  telescope  is  mounted  in  the  shed  between  the  central  dome  and  the  dome 
to  the  left,  which  contains  a  13-inch  and  a  10-inch  photographic  telescope.  The 
east  wing  of  the  main  building  is  to  the  right,  and  is  mainly  devoted  to  the  com- 
puting work  described  below.  The  cottage  from  which  the  picture  is  taken  con- 
tains a  large  dark  room,  in  which  all  the  photographs  are  developed,  an  adjoining 
room  for  general  experimenting,  a  room  for  clerical  work,  and  a  room  which 
may  be  darkened  and  light  thrown  in  by  a  mirror ;  this  room  is  used  for  enlarge- 
ments and  other  experimental  work. 

An  important  advance  has  been  made  by  the  recent  improvements  in  the 
manufacture  of  dry  plates.  The  M.  A.  Seed  Company  of  St.  Louis  have  endeavored 
to  comply  with  our  request  for  more  sensitive  plates,  and  have  gradually  increased 
their  sensitiveness,  so  that  they  now  furnish  us  with  plates  measuring  27  on 
their  scale,  while  a  year  ago  the  most  sensitive  plates  were  only  numbered  21.  As 
a  result,  stars  nearly  a  magnitude  fainter  can  be  photographed,  and  the  number 
of  objects  which  can  be  examined  is  nearly  doubled.  A  careful  study  will  shortly 
be  made,  by  the  help  of  the  instruments  described  below,  of  the  most  sensitive 
plates  obtainable.  It  is  hoped  that  makers  of  very  sensitive  plates  will  send 
specimens  to  Cambridge  for  trial.  The  demand  for  increased  sensitiveness  is  so 
great  not  only  here,  but  at  all  other  observatories  where  stellar  photography  is 
carried  on,  that  a  real  improvement  would  be  widely  appreciated. 

Various  improvements  have  been  made  in  the  methods  of  detecting  defects 
in  the  photographic  processes.  Each  plate,  when  it  is  taken  from  its  box,  is  ex- 
posed to  a  standard  light  for  exactly  one  second.  The  portion  of  the  flame  of  an 
oil  lamp  shining  through  a  small  circular  aperture  constitutes  the  standard  light. 
The  exposure  is  made  for  a  second  by  means  of  a  pendulum,  which  allows  the 
light  to  shine  on  the  plate  for  this  interval  through  a  small  square  aperture. 
When  the  plate  is  developed,  a  dark  square  appears  near  its  edge,  whose  intensity 
measures  the  sensitiveness  of  the  plate,  and  also  serves  to  detect  any  defect  in  its 
development.  Passing  clouds,  or  a  variation  in  the  clearness  of  the  sky,  are  de- 
tected by  an  instrument  called  the  Pole-star  recorder.  It  consists  of  a  telescope 
with  a  focal  length  of  about  three  feet,  placed  parallel  to  the  earth's  axis.  An 
image  of  the  Pole-star  is  formed  by  it,  and  allowed  to  fall  upon  a  sensitive  plate, 
describing  an  arc  of  a  circle,  which  is  interrupted  whenever  clouds  pass.  The  plate 
is  changed  every  day,  and  the  instrument  is  closed  automatically  by  an  alarm  clock 
every  morning  before  the  twilight  begins.  Much  trouble  is  experienced  from  the 
deposition  of  moisture  on  the  objectives  of  the  photographic  telescopes,  on  account 
of  their  exposure  to  a  large  portion  of  the  sky.  The  failure  of  some  of  the 


SECOND   ANNUAL   REPORT.  5 

earlier  plates  may  be  due  to  this  cause.  Moisture  is  now  carefully  looked  for,  and, 
if  detected,  removed  by  gently  heating  the  objectives.  Another  test  of  the  quality 
of  the  plates  consists  in  occasionally  exposing  a  plate  in  the  8-inch  telescope  to 
the  circumpolar  sky,  first  with  and  then  without  the  prism.  The  trails  of  the  stars 
near  the  pole  and  the  spectra  of  the  brighter  stars  are  thus  photographed.  A  com- 
parison of  the  intensity  of  these  images  tests  the  condition  of  the  air,  the  in- 
strument, and  the  plates. 

The  various  investigations  will  now  be  described  in  order,  as  in  the  last 
Report. 

1.  Catalogue  of  Spectra  of  BrigM  Stars.  —  The  spectra  of  all  the  brighter  stars 
have  been  photographed  with  the  8-inch  telescope,  giving  an  exposure  of  at  least 
five  minutes  to  each.  Each  plate  contains  from  two  to  four  regions  ten  degrees 
square.  The  plates  representing  the  region  north  of  — 25°  were  divided  into 
three  series,  which  may  be  distinguished  as  polar,  zenith,  and  equatorial.  Each 
region  is  contained  on  two  plates,  and  the  work  has  been  repeated  in  two  succes- 
sive years,  so  that  at  least  four  photographs  should  be  obtained  of  all  the  brighter 
stars.  If  a  plate  proved  poor,  it  was  repeated,  so  that  the  very  bright  stars  will 
appear  in  several  plates.  The  photographic  portion  of  this  work  was  finished 
last  November.  If  no  plates  had  been  repeated,  36  polar,  72  zenith,  and  72  equa- 
torial plates  would  have  been  required  each  year,  or  360  in  all.  The  actual 
numbers  of  plates  taken  and  measured  were  46,  120,  and  93,  total  259,  the  first 
year;  and  61,  209,  and  104,  total  374,  the  second  year;  or  633  in  all.  In  the 
later  work  the  number  of  zenith  plates  was  doubled,  to  avoid  the  confusion  arising 
when  several  exposures  were  made  on  a  single  plate.  The  numbers  of  spectra 
measured  on  these  plates  were  2,381,  3,314,  and  2,618,  total  8,313,  the  first  year; 
and  7,199,  8,217,  and  4,074,  total  19,490,  the  second  year.  Two  plates  covering 
the  immediate  vicinity  of  the  north  pole  contain  150  spectra.  The  whole  number 
of  spectra  is  therefore  27,953.  The  measurement  and  identification  of  this  large 
number  of  spectra  has  occupied  the  greater  portion  of  the  time  of  the  corps  of 
computers.  Each  plate  to  be  measured  was'  placed  on  a  stand,  and  the  light  of  the 
sky  was  reflected  through  it  by  means  of  a  mirror.  The  approximate  co-ordinates 
of  each  spectrum  in  turn  were  then  read  off,  and  a  careful  description  of  the  spec- 
trum was  given.  Besides  the  usual  division  into  types,  each  additional  line  visible 
was  recorded  both  as  regards  its  position  and  intensity.  The  photographic  intensity 
of  the  brighest  portion  of  each  spectrum  was  also  measured  by  means  of  a  photo- 
graphic plate,  dark  at  one  end  and  light  at  the  other,  like  a  wedge  of  shade  glass. 


6  HENRY    DRAPER   MEMORIAL. 

When  the  spectra  show  sudden  changes  in  brightness,  additional  measurements 
are  made.  This  portion  of  the  work  is  complete  only  for  the  polar  plates  and 
about  62  of  the  other  plates,  including  12,574  spectra.  The  identification  of  the 
spectra  is  effected  either  by  computation  from  its  co-ordinates,  or  by  laying  the  plate 
upon  the  maps  of  the  Durchmusterung,  the  scale  being  the  same  for  both.  All  the 
plates  have,  however,  been  checked  by  the  latter  method.  The  names  of  the 
stars  are  then  taken  from  the  Harvard  Photometry,  Uranometria  Argentina,  or 
Durchmusterung,  according  to  their  brightness  and  declination.  Their  places  are 
next  brought  forward  to  1900,  the  epoch  of  the  final  catalogue.  As  the  intensity 
of  the  photograph  of  a  given  spectrum  will  vary  greatly  with  the  sensitiveness  of 
the  plate,  the  clearness  of  the  air,  and  the  rate  of  the  driving  clock,  all  must  be 
reduced  to  the  same  system.  The  scale  of  the  Harvard  Photometry  is  adopted 
for  this  purpose.  The  most  prevalent  spectra  are  those  of  the  first  type,  in  which 
the  K  line  is  too  faint  to  be  visible.  After  applying  a  correction  for  the  declination 
of  the  stars,*  the  brightness  of  all  such  spectra  on  each  plate  is  compared  with  the 
photometric  magnitudes.  A  correction  is  thus  derived  for  each  plate,  which  is 
applied  to  all  the  spectra  upon  it.  The  effect  of  color,  so  far  as  it  varies  with  the 
type  of  spectrum,  is  thus  eliminated.  It  is  possible  that,  owing  to  variations  in 
temperature,  or  other  causes,  some  stars  may  be  redder  or  bluer  than  others  having 
the  same  type  of  spectrum. 

2.  Catalogue  of  Spectra  of  Faint  Stars.  —  Until  the  photographs  required  for  the 
research  mentioned  above  were  completed,  the  time  of  the  8-inch  telescope  was 
mainly  devoted  to  them.  Since  then  it  has  been  used  principally  in  photographing 
the  fainter  stars.  An  exposure  of  one  hour  is  given  to  each  portion  of  the  sky,  a 
region  ten  degrees  square  being  included  upon  each  plate.  Stars  as  far  south  as 
— 25°  can  be  advantageously  photographed  at  Cambridge,  and  the  plan  proposed 
covers  this  region.  The  plates  overlap,  so  that  the  region  north  of  — 20°  will  appear 
on  at  least  two  plates.  The  southern  stars  are  only  photographed  when  the  sky  is 
unusually  clear.  Each  plate  is  examined,  and,  if  unsatisfactory,  the  work  is  repeated. 
If  all  were  good,  650  plates  would  be  required.  Thus  far,  606  plates  have  been 
taken,  covering  339  of  the  desired  regions.  As  the  time  of  the  computers  has  been 
mainly  devoted  to  the  first  investigation  mentioned  above,  the  greater  portion  of 
these  plates  have  not  been  measured  or  reduced.  The  total  number  measured  is 
105  plates,  containing  6,931  spectra,  of  which  94  plates  and  6,293  spectra  have  been 
reduced.  The  form  of  reduction  and  publication  will  be  similar  to  the  catalogue 
of  bright  stars,  except  that  it  will  be  convenient  to  retain  the  Durchmusterung 
numbers  and  places,  arranging  the  stars  in  the  order  of  the  zones  in  that  catalogue. 


HENRY   DRAPER   MEMORIAL. 


K          H  h 


PLATE  II. 


SECOND   ANNUAL    REPORT.  7 

It  is  hoped  that  the  photographs  for  this  investigation  will  be  nearly  all  taken  by 
the  autumn  of  1888,  and  the  remainder  during  the  following  year.  To  provide  for 
a  possible  increase  in  sensitiveness  of  the  plates,  precedence  is  given  to  those  com- 
pletely covering  the  sky  once,  the  alternate  plates,  covering  the  sky  the  second  time, 
being  taken  later.  The  actual  improvement  in  the  plates  shows  itself  by  an  increase 
in  the  number  of  spectra  in  this  second  series  of  plates.  In  some  cases  over  three 
hundred  stellar  spectra  appear  on  a  single  plate. 

3.  Detailed  Stud//  of  the  Spectra  of  the  Brigliter  Stars.  —  These  spectra  are  obtained 
by  placing  four  prisms,  having  an  angle  of  about  fifteen  degrees  and  each  nearly  a 
foot  square,  over  the  object  glass  of  the  11-inch  telescope,  as  described  in  the  last 
report.     The  increased  sensitiveness  of  the  plates  has  greatly  increased  the  number 
of  stars  bright  enough  to  produce  a  satisfactory  image  in  this  way.     The  white  stars 
of  the  first  type  give  good  images  when  no  brighter  than  the  fourth  magnitude. 
These  spectra  are  about  four  inches  in  length.     An  improvement  has  been  made  in 
the  method  of  enlargement  with  a  cylindrical  lens  described  in  the  last   Report. 
When  such  a  lens  was  used  with  an  enlarging  lens  having  a  small  aperture,  the 
width  of  the  spectrum  was  greatly  reduced ;  with  a  large  aperture,  the  best  definition 
could  not  be  attained.     A  slit  perpendicular  to  the  axis  of  the  cylindrical  lens  is 
accordingly  placed  over  it.     This  reduces  the  aperture  in  one  direction  so  that  the 
definition  of  the  lines  is  good,  without  affecting  the  width  of  the  spectrum.     Slow 
plates   are   also   used   in   the   enlargements   to  increase  the  contrast.     Much   more 
brilliant  spectra  are  thus  obtained. 

4.  Faint  Stellar  Spectra.  —  As  stated  above,  the  28-inch  reflector  constructed  by 
Dr.  Draper  is  now  ready  for  use.     The  difficulties  commonly  encountered  in  the 
use  of  a  large  reflector  have  been  met,  and  it  is  hoped  successfully  overcome.     A 
spectroscope  has  been  devised  for  this  instrument  which  will  give  a  dispersion  about 
equal  to  that  employed  in  the  first  and  second  of  the  researches  described   above. 
As  the  area  of  the  aperture  of  this  telescope  is  about  eleven  times  that  of  the  8-inch 
telescope,  it  is  hoped  that  much  fainter  stars  can  be  photographed  with  it.     A  study 
will  be  made  of  the  spectra  of  the  variable  stars  of  long  period,  of  the  banded  stars, 
and  of  other  objects  having  peculiar  spectra. 

But  little  progress  has  been  made  with  the  other  investigations  proposed,  includ- 
ing the  reduction  to  wave-lengths,  and  the  study  of  the  approach  and  recession  of 
the  stars.  It  seemed  best  to  concentrate  our  work  on  the  researches  described  above, 
undertaking  the  other  investigations  as  soon  as  time  permitted. 

The  investigations  described  above  are  illustrated  by  Plate  II.  A  special  study 
was  made  of  the  spectrum  of  the  variable  star  /?  Persei.  A  variation  in  this 


8  HENRY   DEAFER   MEMORIAL. 

spectrum  would  have  an  important  bearing  on  the  theory  that  the  diminution 
in  light  is  due  to  an  interposed  dark  satellite.  Spectra  of  this  star  at  minimum 
were  first  obtained  with  one  prism.  With  the  increased  sensitiveness  of  the  plates, 
more  prisms  were  tried,  until  finally  good  spectra  were  obtained  with  all  four  prisms 
even  when  the  star  was  at  its  minimum.  At  first  it  was  thought  that  a  variation 
was  detected  in  the  spectrum,  but  this  change  was  not  confirmed  under  more 
favorable  circumstances.  The  upper  part  of  Figure  1  shows  the  spectrum  of  this 
star  on  February  6,  1888,  when  at  its  full  brightness;  the  lower  part  shows  the 
spectrum  on  February  9,  1888,  when  the  star  was  at  its  minimum.  A  careful 
inspection  of  the  original  negatives  failed  to  show  any  differences  in  the  spectra. 
Twenty  lines  are  visible  at  minimum,  all  of  which  are  seen  at  maximum.  The 
difference  in  brightness  of  the  ends  of  the  spectrum  is  a  photographic  effect,  not 
shown  iu  the  original  negative.  The  spectrum  of  a  Orionis  is  shown  in  Figure  2. 
Before  the  recent  increase  in  the  sensitiveness  of  the  photographic  plates,  satisfac- 
tory photographs  could  not  be  obtained  of  the  spectrum  of  this  star,  on  account 
of  its  red  color. 

The  method  of  testing  plates  by  exposing  them  to  the  polar  sky,  as  described 
above,  is  illustrated  in  Figure  3.  The  spectrum  is  that  of  the  Pole-star.  Although 
no  clockwork  is  used,  DM.  +88°4,  magnitude  6.5,  leaves  a  distinct  spectrum  by 
trailing  on  the  most  sensitive  plates.  The  short  lines  are  the  trails  when  no  prism 
is  used.  Figure  4  represents  the  record  of  cloudiness  on  December  28,  1887,  by  the 
trail  of  the  Pole-star.  The  breaks  in  the  circular  trail  indicate  the  passage  of  clouds. 
From  it  we  see  that  for  about  five  minutes  after  7A  45m,  when  the  record  begins, 
the  light  was  slightly  dimmed  by  clouds.  It  was  then  clear  until  9A  Om ;  cloudy  until 
9*  1(T;  clear  until  9*  40™ ;  cloudy  at  times  until  11*  0'";  clear  until  17A  35ro,  when 
the  apparatus  was  closed  automatically  by  the  alarm  clock.  The  square  formed  on 
this  plate  by  the  standard  light  is  shown  in  Figure  5. 

A  means  of  studying  the  steadiness  of  the  air  and  the  twinkling  of  the  stars  is 
shown  in  Figure  6.  The  8-inch  telescope  was  directed  to  a  Lyrce  on  October  17,  1887, 
and  the  spectrum  of  this  star  was  allowed  to  trail  across  the  plate,  the  telescope 
being  at  rest.  The  lines  G,  I,  and  H  appear  as  nearly  vertical  lines.  If  the  air 
was  perfectly  steady,  a  uniform  band  would  be  obtained.  The  irregularities  of  the 
air,  which  appeared  perfectly  clear  to  the  eye,  are  shown  in  the  upper  part  of 
the  band.  They  increase  in  the  lower  part,  when  the  star  was  obscured  by  passing 
clouds. 


HENRY  DRAPER  MEMORIAL. 


THIRD    ANNUAL   REPORT 


OF    THE 


PHOTOGRAPHIC  STUDY  OF  STELLAR  SPECTRA 


CONDUCTED    AT   THE 


HARVARD  COLLEGE  OBSERVATORY, 


EDWARD    C.    PICKERING,   DIRECTOR. 


CAMBRIDGE: 

JOHN     WILSON"    AND     SON, 
35nibtrsitg 

1889. 


HENRY  DRAPER  MEMORIAL. 


THIRD   ANNUAL   REPORT 


OF    THE 


PHOTOGRAPHIC  STUDY  OF  STELLAR  SPECTRA 


CONDUCTED   AT   THE 


HARVARD  COLLEGE  OBSERVATORY, 


EDWARD    C.   PICKERING,  DIRECTOR. 


UITIVEESITY 


CAMBRIDGE: 

JOHN   WILSON    AND    SON, 

Slmbrrsitu  ^Jrrss. 

1889. 


f^N 

[WIVEHSITY] 


HENRY   DRAPER    MEMORIAL. 


THIRD   ANNUAL   REPORT. 

THE  researches  which  constitute  the  Henry  Draper  Memorial  have  consisted  for 
the  last  three  years  in  the  photographic  study  of  the  spectra  of  the  stars.  While 
this  subject  will  continue  to  be  the  principal  one  under  investigation,  Mrs.  Draper 
has  decided  to  extend  the  field  of  work  undertaken,  so  as  to  include  the  study 
of  the  other  physical  properties  of  the  stars  by  photography.  As  will  be  seen 
from  the  detailed  statement  below,  the  first  research  undertaken  is  now  rapidly 
approaching  completion,  the  plans  for  the  study  of  the  southern  stars  have  been 
matured,  and  this  study  will  soon  be  begun;  the  detailed  study  of  the  spectra  of 
the  brighter  stars  is  making  progress,  and  a  large  piece  of  photometric  work  will 
soon  be  undertaken  with  a  new  telescope.  The  progress  made  in  each  investigation 
will  now  be  described,  as  in  previous  reports. 

1.  Catalogue  of  Spectra  of  Bright  Stars.  —  The  Bache  telescope,  which  has  an 
8-inch  photographic  doublet  as  an  objective,  is  used  for  this  work.  The  photographs 
cover  the  entire  sky  north  of  — 25°,  with  exposures  of  about  five  or  ten  minutes. 
About  28,000  spectra  of  10,800  stars  have  been  examined,  including  nearly  all 
stars  visible  in  Cambridge  of  the  seventh  magnitude  or  brighter.  The  catalogue  is 
now  nearly  ready  for  the  printer,  the  final  copy  having  been  prepared  as  far  as 
14*  in  right  ascension. 

Nearly  the  entire  time  of  three  or  four  computers  has  been  devoted  during  the 
past  year  to  this  work.  The  intensity  of  about  15,000  of  the  spectra  has  been 
measured,  completing  this  part  of  the  research.  Much  time  has  been  spent  in 
checking  and  verifying  the  results.  All  the  positions  have  been  checked  and 
brought  forward  to  1900  two  or  more  times  independently.  All  discordant  measures 
have  been  re-examined,  and  a  search  has  been  made  for  possible  error  when  bright 
stars  are  omitted  or  very  faint  ones  inserted.  Seven  thousand  two  hundred  notes 
have  been  made  on  the  various  stars  in  the  catalogue.  Each  note  has  an  appro- 


4  HENRY   DRAPER   MEMORIAL. 

priate  number  which  permits  it  to  be  entered  in  its  proper  place.  Most  of  these 
notes  relate  to  additional  lines  contained  in  these  spectra  besides  those  by  which 
the  type  is  determined.  The  position  and  intensity  of  these  lines  is  estimated.  A 
portion  of  them  have  been  reduced  to  wave-lengths.  The  printing  of  the  catalogue 
might  have  been  already  begun,  but  for  the  difficulty  of  deciding  how  the  different 
types  of  spectra  should  be  distinguished.  The  classification  used  for  visual  obser- 
vations fails  to  indicate  many  differences  obvious  in  the  photographs.  On  the 
other  hand,  the  photographic  portions  of  spectra  of  Types  II.  and  III.  are  nearly 
identical.  The  photographs  also  show  many  stars  whose  spectra  are  intermediate 
between  those  of  the  typical  stars  which  have  determined  the  usual  classification. 
A  system  has,  however,  been  adopted  which  permits  all  differences  detected  in 
the  photographs  to  be  described  in  the  printed  volume. 

Thirteen  spectra  were  found  on  these  plates  which  could  not  be  identified 
with  stars.  Three  of  these  proved  to  be  due  to  Mars,  one  to  Vesta,  three  to 
Jupiter,  four  to  Saturn,  and  two  to  Uranus.  Accordingly  all  the  exterior  planets 
bright  enough  to  be  detected  in  this  way  appear  on  these  plates. 

The  measures  of  the  intensity  of  the  spectra  form  a  very  important  portion  of 
this  work.  Since  the  same  part  of  the  spectrum  is  measured  in  each  case,  the  true 
relative  energy  is  determined.  That  is,  the  same  result  is  obtained  as  if  the  meas- 
ures of  rays  of  the  same  wave-length  were  made  by  any  other  method,  as  photo- 
metrically by  the  eye,  by  the  thermopile,  or  by  the  bolometer.  The  color  of  the 
star  will  be  indicated  by  the  extent  of  the  spectrum,  which  is  also  noted.  For  the 
first  time,  therefore,  we  shall  have  a  photometric  catalogue  in  which  the  error  due  to 
the  color  of  the  star  is  eliminated.  A  preliminary  determination  of  the  accordance 
of  the  results  derived  from  different  photographs  of  the  same  star  shows  that  the 
average  value  of  the  residuals  will  be  about  0.16,  which  is  the  same  as  the  corre- 
sponding quantity  for  the  Harvard  Photometry.  The  number  of  stars  is  more 
than  twice  that  contained  in  the  latter  Catalogue. 

2.  Catalogue  of  Spectra  of  Faint  Stars.  —  In  November,  1888,  the  photographs 
required  to  cover  the  sky  north  of  the  equator  were  nearly  finished.  It  was  ex- 
pected that  in  two  months  the  observations  would  be  completed.  The  telescope, 
which  was  the  same  as  that  used  in  the  previous  research,  was,  however,  wanted 
for  photographing  the  Solar  Eclipse  of  January  1,  1889.  It  was  accordingly  sent 
to  Willows,  California,  where  it  was  mounted,  and  the  greater  portion  of  the  re- 
maining photographs  were  taken  there.  It  was  then  sent  to  Peru,  as  will  be 
described  below.  The  few  remaining  photographs,  including  the  repetition  of  those 
found  on  further  examination  to  be  unsatisfactory,  will  be  taken  in  Peru. 


THIRD   ANNUAL   REPORT. 

3.  Detailed  Study  of  the  Spectra  of  the  Brighter  Stars.  —  The  11-inch  refractor 
with  one,  two,  or  four  large  prisms  over  its  objective  has  been  employed  in  this  work 
throughout  nearly  every  clear  night,  until  stopped  by  the  morning  twilight;  686 
photographs  have  been  taken,  most  of  them  with  an  exposure  of  two  hours.  With 
our  present  photographic  plates  about  570  stars  north  of  —30°  are  bright  enough 
to  be  photographed  with  one  prism,  170  of  them  with  two  prisms,  and  87  of  them 
with  four  prisms.  To  obtain  the  best  possible  result  some  of  the  photographs  must 
be  repeated  many  times.  The  difficulty  is  increased  by  the  invariably  hazy  appear- 
ance of  the  lines  in  some  spectra,  like  that  of  a  Aqnilce,  which  was  at  first  attributed 
to  poor  definition  of  the  photograph.  It  is  expected  that  the  work  will  be  com- 
pleted during  the  next  year  by  original  or  repeated  photographs  of  228  stars 
with  one  prism,  of  64  with  two,  and  of  12  with  four.  In  general,  stars  as  bright  as 
the  fourth  magnitude  can  be  satisfactorily  photographed  with  one  prism,  the  spectra 
obtained  being  about  an  inch  long.  Fainter  stars,  if  of  a  bluish  color,  give  sufficiently 
distinct  images,  in  some  cases  good  results  being  obtained  with  stars  of  the  seventh 
magnitude.  For  example,  fourteen  stars  in  the  Pleiades  are  well  photographed  with 
this  apparatus.  With  four  prisms  much  longer  spectra  are  obtained  and  many  more 
lines  are  visible.  But  certain  differences  in  the  character  of  the  spectra  are 
better  shown  with  the  smaller  dispersion.  Numerous  photographs  have  been  taken 
of  the  variable  stars  o  Ceti  and  /3  Lyrce.  The  changes  in  the  spectrum  of  the 
latter  star  seem  to  be  undoubted,  those  of  o  Ceti,  if  any,  to  be  slight.  Various 
peculiarities  in  the  spectra  of  individual  stars  have  been  detected.  One  photograph 
of  £  UrscB  Majoris  shows  the  K  line  distinctly  double,  and  others  show  it  single. 
Many  photographs  will  be  required  to  determine  the  law  of  its  variation,  if  this  is 
due  to  changes  in  the  star  itself.  Bright  lines  were  detected  in  the  spectrum  of 
<f>  Persei,  putting  it  in  a  class  in  which  only  two  or  three  other  stars  are  known  to 
fall.  In  the  double  star  ft  Cygni  the  two  components  have  spectra  of  different 
types,  an  important  consideration  in  the  theories  regarding  their  formation.  The 
brighter  component  is  of  the  second  type,  the  fainter  of  the  first. 

Ordinary  photographic  plates  are  not  sensitive  to  rays  of  much  greater  wave- 
length than  the  F  line,  or  486.  By  staining  the  plates  with  various  coal-tar  products 
the  range  of  sensitiveness  may  be  greatly  extended.  With  erythrosin  the  spectrum 
extends  to  the  wave-length  590.  The  sodium  line  D  is  distinctly  seen  to  be  double 
in  the  photographs  of  a  Bootis  and  a  Aurigce.  Various  experiments  were  also  made 
with  cyanin,  but  the  plates  were  not  sufficiently  sensitive  to  give  good  results.  The 
entire  length  of  the  spectrum  with  four  prisms,  including  the  portion  obtained  by 
erythrosin,  is  about  six  inches  and  a  half. 


6  HENRY    DRAPER   MEMORIAL. 

A  beginning  has  been  made  of  the  measures  of  the  positions  of  the  lines  in  the 
spectrum.  A  scale  of  fortieths  of  an  inch  has  been  ruled  on  glass,  and  the  positions 
of  the  lines  read  off  with  the  aid  of  a  magnifying-glass.  Twelve  of  the  photographs 
of  a  Cants  Majoris  have  been  studied  in  this  way.  The  spectrum  of  this  star  is 
traversed  by  the  hydrogen  lines,  which  are  strong,  and  by  other  lines  which  are  so 
faint  that  they  are  only  visible  when  the  dispersion  is  large  and  the  definition  good. 
The  catalogue  thus  formed  contains  about  320  lines.  The  average  deviation  of  the 
measures  of  the  same  line  on  different  plates  is  about  0.05  of  a  millionth  of  a 
millimetre,  or  0.05  cm.  on  the  scale  of  Angstrom's  map.  If  the  line  occurs  in  the 
solar  spectrum  these  measures  will  generally  identify  it.  In  other  cases  the  exact 
position  must  be  determined  by  a  dividing  engine.  If  a  line  can  be  distinctly  seen, 
its  wave-length  can  probably  be  thus  determined  with  as  great  accuracy  as  that  of 
the  position  of  the  solar  lines  on  the  map  of  Angstrom.  In  the  spectrum  of  a  Bootis 
140  lines  are  visible  between  the  D  and  F  lines. 

The  classification  of  this  large  number  of  spectra  is  a  matter  of  no  little  difficulty. 
Slight  differences  exist  in  many  stars,  and  certain  stars  appear  to  hold  an  interme- 
diate position,  so  as  to  render  a  rigorous  division  into  classes  impossible.  On  the 
other  hand,  many  stars  appear  to  have  identical  spectra.  The  first  step  will  be  to 
arrange  the  stars  in  groups,  and  then  compare  the  best  defined  spectra  of  different 
groups.  A  minute  discussion  and  the  measurement  of  wave-lengths  will  be  necessary 
only  in  the  investigation  of  a  comparatively  small  number  of  spectra. 

4.  Faint  Stellar  Spectra.  —  The  28-inch  reflecting   telescope   constructed   by  Dr. 
Draper  was  assigned  to  this  work.     During  the  first  six  months  of  the  year  a  careful 
study  was  made  of  this  problem,  and  the  difficulties  encountered  bore  evidence  of  the 
skill  of  Dr.  Draper  in  obtaining  good  results  with  this  telescope.     The  best  method 
of  using   this  instrument  seemed   to  be  a  modification  of  the  form  first  tried  by 
Dr.  Draper,  —  a  slit  spectroscope  from  which  the  slit  had  been  removed.     The  rays 
from  the  mirror  were  rendered  parallel  by  a  concave  lens  which  replaced  the  objec- 
tive of  the  collimator.     As  this  lens  had  the  same  focal  distance  as  the  objective  of 
the  observing  telescope,  it  was  not  necessary  that  either  should  be  achromatic.     After 
long  trials  with  this  and  other  forms  of  apparatus,  a  spectrum  was  at  length  obtained 
showing  good  definition.     As  the  results  were  not  better  than  those  described  above, 
and  the  instrument,  from  its  size,  was  slow  in  operation,  the  experiments  have  not 
been  carried  further. 

5.  Catalogue  of  Spectra  of  BiigU  Southern  Stars.  —  The   8-inch  Bache    telescope 
remained  in  California  until  February  2,  1889,  and  was  then  sent  to  Peru  to  continue 
research  No.  1  on  the  southern  stars.     The  sky  from  — 25°  to  the  south  pole  will 


TIIIKD   ANNUAL    REPORT.  7 

be  covered,  and  the  resulting  photographs  sent  to  Cambridge  and  reduced,  as  in  the 
case  of  the  northern  stars.  The  advantages  of  discussing  all  stars  from  the  north  to 
the  south  pole  according  to  one  system  are  very  great,  and  are  here  secured  for  the 
first  time  in  so  extensive  an  investigation.  If  no  unforeseen  difficulty  arises,  the 
photographs  will  all  be  completed  during  the  next  two  years. 

6.  Catalogue  of  Spectra  of  Faint  Southern  Stars.  —  Research  No.  2  will  also  be 
extended  to  the  south  pole  simultaneously  with  the  observations  required  for  No.  5. 
It  is  expected  that  these  photographs  also  will  be  completed  in  two  years. 

The  Bache  telescope  described  above  has  proved  an  extremely  convenient  instru- 
ment for  various  purposes.  Besides  the  spectroscopic  researches  already  mentioned, 
several  other  investigations  have  been  undertaken  with  it,  some  of  which  will  be  found 
in  the  Memoirs  of  the  American  Academy,  Vol.  XL,  p.  179,  and  the  Harvard  Observa- 
tory Annals,  Vol.  XVIII.,  Nos.  IV.,  VI.,  and  VII.  Owing  to  its  short  focal  length  it 
possesses  many  advantages  over  photographic  telescopes  of  the  usual  form.  With  ex- 
posures of  an  hour  and  a  half  more  stars  were  photographed  in  the  Pleiades  than  are 
given  in  the  engraving  accompanying  the  Annual  Report  of  the  Paris  Observatory 
for  1886,  although  that  work  was  based  on  photographs  taken  by  the  MM.  Henry  with 
exposures  of  three  hours  and  a  telescope  having  an  aperture  of  13  inches.  Nearly 
twice  as  many  stars  were  photographed  in  this  region  as  were  visible  with  the 
15-inch  telescope  of  the  Harvard  College  Observatory.  The  short  focus  of  the 
telescope  also  gives  it  especial  advantages  for  photographing  nebulae.  Twelve  new 
nebula)  were  thus  discovered  in  a  region  where  but  eighteen  were  known  before. 
Various  other  investigations,  such  as  a  determination  of  the  law  of  atmospheric 
absorption,  have  been  undertaken  with  the  aid  of  this  telescope.  It  has  been  so 
persistently  used  in  spectroscopic  work  that  the  other  researches  have  been  neglected, 
especially  those  in  which  very  long  exposures  were  required.  Its  removal  to  Peru 
now  cuts  it  off  for  some  time  from  such  use  on  the  northern  stars.  Accordingly, 
Mr*.  Draper  has  procured  a  similar  lens,  which  is  now  in  the  hands  of  the  firm  of 
Alvan  Clark  &  Sons  for  retouching  and  mounting.  Several  important  researches 
will  be  undertaken  with  this  instrument.  Photography  is  now  used  in  so  many 
departments  of  astronomy  that  a  general  investigation  of  the  photographic  brightness 
of  the  stars  seems  desirable.  A  plan  has  been  proposed  by  which  a  single  plate  will 
contain  photographs  of  a  number  of  regions  one  degree  square,  but  in  different 
portions  of  the  sky.  Thus  a  series  of  standard  faint  stars  will  be  photographed, 
which  can  all  be  measured,  and  reduced  to  the  same  scale.  One  or  more  photo- 
graphs of  the  vicinity  of  the  north  pole  will  be  taken  on  each  plate,  and  thus  serve 


HENRY   DRAPER   MEMORIAL. 

to  correct  the  results  obtained  on  different  plates.  It  is  proposed  in  this  way  to  secure 
a  series  of  standards  of  stellar  magnitude  at  intervals  of  about  five  degrees.  A  third 
lens  of  similar  form,  having  an  aperture  of  four  inches,  will  be  attached  to  the  tele- 
scope, with  which  photographs  on  a  smaller  scale,  but  five  degrees  square,  will  be 
taken  simultaneously.  These  photographs  will  cover  the  entire  sky,  and  it  is  proposed 
to  measure  the  photographic  brightness  of  all  stars  of  the  seventh  magnitude,  or 
brighter,  which  are  represented  on  them.  This  investigation  will  have  a  special 
value  in  connection  with  the  photometric  measures  of  the  spectra  described  above. 
It  is  hoped  also  to  photograph  the  entire  northern  sky  by  means  of  the  8-inch 
telescope,  with  exposures  of  an  hour.  Each  plate  covers  a  region  nearly  ten  de- 
grees square,  of  which  the  images  in  the  central  five  degrees  square  are  satisfac- 
torily in  focus.  One  of  the  regions  containing  standard  stars  will  appear  in  the 
centre  of  each  plate.  By  such  a  series  of  plates  the  photographic  brightness  of  any 
stars  brighter  than  the  fifteenth  magnitude  can  be  determined  on  a  uniform  scale. 
The  faintest  stars  photographed  will  be  nearly  a  magnitude  fainter  than  the  limit 
proposed  by  the  Astrophotographic  Congress,  so  that  all  plates  included  in  that  work 
can  be  reduced  to  a  uniform  system.  The  advantages  of  such  plates  for  studies  of 
the  distribution  of  the  stars  and  other  similar  investigations  are  obvious. 

From  the  above  description  it  appears  that  the  field  of  work  of  the  Henry  Draper 
Memorial,  as  now  extended,  is  almost  boundless.  The  problems  to  be  investigated 
relate  to  the  fundamental  laws  regulating  the  formation  of  the  stellar  system. 
Questions  of  such  importance  should  be  discussed  on  a  sufficiently  large  scale,  or 
the  results  of  the  discussion  will  soon  be  superseded  by  a  repetition  of  the  work. 
The  liberal  provision  made  for  the  Henry  Draper  Memorial  permits  the  investigations 
to  be  planned  on  a  scale  which  is  likely  to  avoid  such  an  undesirable  duplication  of 
work. 


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